Conventionally, in a three-dimensional installation of semiconductor package, coupling between a semiconductor chip and a semiconductor chip or an interposer is performed by wire bonding. In place of the wire bonding, a three-dimensional installation technique, with which semiconductor chips are coupled with each other via a pass-through electrode and a bump, has been developed. The pass-through electrode normally has a short connecting wire length (for example, 50 μm), and a bump, via which electrodes are connected, is also required to be microscopic. A technique that handles such a bump pitch of less than 50 μm is referred to as a microbump. As U.S. Pat. No. 9,136,159, a wiring length between semiconductor chips can be dramatically shortened by coupling the semiconductor chip and the semiconductor chip with the pass-through electrode and the microbump. Accordingly, a wiring delay time that increases with miniaturization can be reduced.
Forming processes of the microbump include a solder printing method, a solder ball mounting, and a plating method, where void generated by gas, etc. generated in a reaction process of an organic substance, water, etc. becomes an issue. In this context, Japanese Patent No. 5807221 states performing reflow in an inert atmosphere and reflow in a reducing atmosphere of hydrogen, carboxylic acid, etc.